All-metal tanks (e.g., stainless steel tanks) have been widely used for storing and transporting various chemicals and chemical compositions. All-metal tanks can readily be formed which will accommodate internal vacuum and/or high pressure conditions. All-metal tanks can also be readily formed which are sturdy enough to withstand the general mechanical stresses encountered in loading, unloading, stacking, transporting, minor collisions, etc.
Unfortunately, all-metal tanks are generally very heavy and expensive to build. In many applications (e.g., the storage and/or transportation of acids and other well treating fluids), all-metal tanks are also susceptible to internal corrosive chemical attack. Although, in such applications, certain metallic materials of construction can often be selected which are compatible with the corrosive chemical compositions being stored and/or transported, such materials of construction can be exotic and/or expensive.
Certain elastomers, polyolefins, rubber materials, and the like are highly resistant to corrosive chemical attack. Such materials are readily available, are lightweight, and can be comparatively inexpensive. However, tanks formed from these materials are generally susceptible to mechanical damage.
To alleviate the above-mentioned problems encountered in the use of all-metal tanks and to obtain the advantages of both the metallic and the nonmetallic materials of construction mentioned above, lined metal tanks have been produced which are compatible with the chemical compositions being stored and/or transported therein. These lined tanks have generally consisted of either (1) a metallic tank having a lining adhered to its interior surface or (2) a metallic tank having a thin, flexible bladder inserted therein.
A plastic or rubber lining material can be applied to the interior wall of a metal tank by securing sheets of the lining material to the interior wall using a bonding agent. However, when a lining material is bonded to the interior wall of a metallic tank in this manner, the resulting liner includes undesirable seams, joints, and other regions of non-uniform thickness.
A liner can also be secured to the interior wall of a metal tank by spraying a liquid plastic material thereon. However, when a liquid lining material is sprayed onto the interior surface of a metal tank, it is difficult to ensure that the resulting liner will be free of voids, pinholes, or other defects. Such defects leave the metallic shell exposed to corrosive chemical attack.
It is also noted that, whenever a lining is attached to the interior surface of a metallic tank by either bonding or spraying, tearing or splitting of the liner can occur as a result of thermal expansion and contraction due to the disparate coefficients of thermal expansion of the liner and the metallic shell. An attached liner is also easily ruptured by physical blows to the exterior of the metallic shell.
Tanks having bladder-type plastic linings, on the other hand, have proven to be unsatisfactory for most applications. Thin, flexible, plastic bladders can be easily damaged even during routine filling, handling, and cleaning operations.
U.S. Pat. No. 4,625,892 discloses a tank system composed of a rigid, linear low density polyolefin tank mounted within a metallic tank. The polyolefin tank is not bonded to the interior wall of the metallic tank. Since, for the most part, the inner polyolefin tank and the outer metallic tank are free to expand and contract independently of each other, the amount of stress imparted to the plastic tank as a result of thermal expansion and contraction is greatly reduced. Further, since the inner plastic tank is not adhered to the outer metallic tank, the likelihood that the inner plastic tank will be damaged by a blow to the exterior of the metallic tank is reduced.
The tank system of U.S. Pat. No. 4,625,892 is produced by forming a linear, low density, polyolefin tank inside an outer metallic tank using a rotational molding (rotomolding) technique. This rotational molding technique includes the steps of: (1) applying a high temperature paint or other release agent to the inner walls of the metallic tank; (2) heating the metallic tank; (3) placing a polyolefin powder (or liquid) inside the heated metallic tank; (4) rotating and continuing to heat the metallic tank such that the polyolefin powder melts and coats the inner walls of the metallic tank; (5) cooling the metallic tank so that the plastic tank formed therein solidifies and shrinks away from the interior surface of the metallic tank; and (6) during the cooling cycle, pressurizing the interior of the plastic inner tank such that the amount of inner tank shrinkage produced during the cooling cycle is minimized.
As part of the production method of U.S. Pat. No. 4,625,892, fixtures are also attached to the flanges and other connections of the metallic tank so that the melted polyolefin material is allowed to flow through the tank fittings and to coat one or more of the outer surfaces of the metallic tank connections. Thus, complete polyolefin connections corresponding to the connections of the metallic tank are formed in the inner polyolefin tank during the rotational molding process. To ensure that the outer metallic material is completely isolated from the contents of the inner plastic tank, the inner tank connections can be sealed along with the connections of the outer metallic tank using polyolefin-lined closures.
Unfortunately, the tank system of U.S. Pat. No. 4,625,896 has substantial shortcomings. The plastic connections formed in the inner plastic tank during the rotomolding process of U.S. Pat. No. 4,625,896 cover and extend over the connections of the outer metal tank. Due to the resulting intimate relationship between the connections of the plastic tank and the metallic tank, the connection areas of the plastic tank are subjected to substantial stress when the plastic tank shrinks away from the outer metal tank during the cooling cycle of the rotomolding process.
Substantial stress can also be imparted to the plastic inner tank of U.S. Pat. No. 4,625,892, and particularly to the connection areas of said inner tank, due to movement of the inner plastic tank within the outer metal tank during loading, transporting, unloading, etc. As indicated above, a substantial gap is created between the exterior wall of the plastic tank and the interior wall of the metal tank due to shrinkage during the rotomolding cooling cycle. Particularly large gaps will typically be created between the longitudinal exterior ends of the plastic tank and the longitudinal interior ends of the metal tank. Thus, the plastic tank is undesirably allowed to flex and move within the metal tank when material is added to or taken out of the tank system and when the tank system payload shifts within the plastic tank.
In tank systems of the type described in U.S. Pat. No. 4,625,892, attempts have been made to restrict the movement of the plastic tank within the metal tank by injecting an expanding urethane foam into the gap existing between an exterior end of the plastic tank and the corresponding interior end of the metal tank. In order to allow the injection of the urethane material, a port consisting of an internally threaded pipe collar will typically be provided through the elongate cylindrical portion of the metal tank at a point immediately adjacent the end of the metal tank. After the plastic tank is formed in the metal tank, a hose is fed through the port and down to the bottom of the gap existing between the ends of the tanks. The urethane material is then delivered through the hose as the hose is slowly pulled out of the metal tank.
Unfortunately, although this urethane injection technique has been helpful, the degree to which the plastic tank is allowed to flex and move within the metal tank remains undesirably high.